Re: [PATCH V12] mm/debug: Add tests validating architecture page table helpers

[Anshuman Khandual <anshuman.khandual@xxxxxxx>]

On 02/12/2020 11:25 PM, Gerald Schaefer wrote:
> On Wed, 12 Feb 2020 15:12:54 +0530
> Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
> 
>>>> +/*
>>>> + * On s390 platform, the lower 12 bits are used to identify given page table
>>>> + * entry type and for other arch specific requirements. But these bits might
>>>> + * affect the ability to clear entries with pxx_clear(). So while loading up
>>>> + * the entries skip all lower 12 bits in order to accommodate s390 platform.
>>>> + * It does not have affect any other platform.
>>>> + */
>>>> +#define RANDOM_ORVALUE	(0xfffffffffffff000UL)  
>>>
>>> I'd suggest you generate this mask with something like
>>> GENMASK(BITS_PER_LONG, PAGE_SHIFT).  
>>
>> IIRC the lower 12 bits constrains on s390 platform might not be really related
>> to it's PAGE_SHIFT which can be a variable, but instead just a constant number.
>> But can definitely use GENMASK or it's variants here.
>>
>> https://lkml.org/lkml/2019/9/5/862
> 
> PAGE_SHIFT would be fine, it is 12 on s390. However, in order to be
> more precise, we do not really need all 12 bits, only the last 4 bits.
> So, something like this would work:
> 
> #define RANDOM_ORVALUE GENMASK(BITS_PER_LONG - 1, 4)
> 
> The text in the comment could then also be changed from 12 to 4, and
> be a bit more specific on the fact that the impact on pxx_clear()
> results from the dynamic page table folding logic on s390:
> 
> /*
>  * On s390 platform, the lower 4 bits are used to identify given page table
>  * entry type. But these bits might affect the ability to clear entries with
>  * pxx_clear() because of how dynamic page table folding works on s390. So
>  * while loading up the entries do not change the lower 4 bits.
>  * It does not have affect any other platform.
>  */

Sure, will update accordingly.